linux-kernel - [RFC] PPC: MPIC: necessary readback after EOI?

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Message-ID: <54AA9C4D.3000001@freescale.com>
Date:	Mon, 5 Jan 2015 16:14:37 +0200
From:	Purcareata Bogdan <b43198@...escale.com>
To:	<benh@...nel.crashing.org>
CC:	<linuxppc-dev@...ts.ozlabs.org>, <linux-kernel@...r.kernel.org>
Subject: [RFC] PPC: MPIC: necessary readback after EOI?

Hello,

While doing some performance testing of a KVM guest on a PPC platform, I 
noticed that there's a read of the CPU_WHOAMI register after each MPIC 
EOI [1]. This has been present since the initial implementation of the 
MPIC driver [2]. In a KVM virtualized environment, this results in an 
additional kvm_exit.

Is the read back necessary? Is it used to provide some sort of 
synchronization mechanism, making sure that nothing else is executed 
until the EOI write is finished? I eliminated the mpic_cpu_read call and 
run the kernel on hardware and noticed no anomaly, however I am not sure 
of all the implications and race conditions it might lead to.

I was curious why the mpic_cpu_read(MPIC_INFO(CPU_WHOAMI)) was there in 
the first place and if it's still needed. If it's still required, I 
guess a better approach is to eliminate the call only if the kernel is 
running on the KVM guest side, where the MPIC is emulated and no longer 
requires a readback.

Thank you,
Bogdan P.

[1] http://lxr.free-electrons.com/source/arch/powerpc/sysdev/mpic.c#L659
[2] https://lkml.org/lkml/2004/10/22/483
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